1. Field of the Invention
The present invention relates to a method of manufacturing an insulation substrate for a semiconductor device and a novel metal member used in the manufacturing process.
2. Description of the Prior Art
In a power semiconductor device, a semiconductor chip or element must be fixed therein and electronic connection related to the semiconductor element must be attained. Thus, the device is often provided with an insulation substrate which includes a conductive circuit pattern and an insulation film. FIG. 9A is a plane view of such an insulation substrate of a conventional design. FIG. 9B is a cross sectional view of the same taken along lines IX--IX of FIG. 9A. An insulation substrate 100 includes a metal plate 101 and an insulation film 102 formed thereon. A metal circuit pattern 103 is fixed on the insulation film 102. The circuit pattern 103 consists of three portions 103a, 103b and 103c. On the portion 103b disposed in the center of the three, a semiconductor chip 104 is mounted. The portions 103a and 103c, disposed on the both sides of the portion 103b, are electrically connected to the semiconductor chip 104 by an aluminum wire 105.
The metal plate 101 functions as a heat sink for radiating heat which is generated when the semiconductor chip 104 operates, the thickness of the metal plate 101 being about 1 to 3 mm. The insulation film 102 electrically insulates the circuit pattern 103 and the metal plate 101, and the thickness thereof is about 0.05 to 0.2 mm. The circuit pattern 103 has a thickness of about 0. 03 to 0.3 mm. Through an external terminal erected on the circuit pattern 103 in an edge area 103e thereof, the semiconductor chip 104 and external circuits are electrically connected. The insulation plate 100 is provided with bolt holes 106 through which the semiconductor device is bolted to a desired apparatus.
Recent years have seen a growing demand for a small sized power device which controls large electric power. However, the conventional insulation substrate 100 does not fulfill the demand, since the vertical sectional area of the circuit pattern 103 is not so large, when a large current flows in the semiconductor chip 104, the circuit pattern 103 allows only insufficient horizontal thermal diffusion. In addition, within the pattern circuit 103 itself, increased voltage drop and heat radiation can result. As this result, a semiconductor device comprising the insulation substrate 100 can not withstand large electric power.
One of the methods for increasing the vertical sectional area of the circuit pattern 103 is to enlarge the plane area of the circuit pattern 103. However, this method increases an overall plane area of the insulation substrate 100 accordingly, and is thus, in direct contradiction to the demand for small size and duration to large electric power. It is therefore understood that the circuit pattern 103 must be thickened to increase the vertical sectional area thereof without increasing the size of the device.
As a method for obtaining a thick circuit pattern, two methods are considered. The first method is conducted through selective etching of a uniform and relatively thick metal plate affixed to the insulation film 102. In the second method, parts to form the circuit pattern are manufactured, as discrete components, from a uniform and relatively thick metal plate by punching. Thereafter, thus manufactured parts are arranged and affixed on the insulation film 102.
The first method causes a problem that etching a thick metal plate leads to a low size accuracy. The second method is not problem-free, either. Although parts for the circuit pattern can be sized with a high accuracy, the number of components to be produced separately is increased. In addition, the second method requires means for positioning the parts of the circuit pattern on the insulation film 102. Thus, manufacturing costs would swell up.